To meet the stricter limiting values for pollutant emissions, a variety of measures are used in contemporary engines, in particular to reduce the particle and nitrogen oxide emissions.
One measure is exhaust gas recirculation (EGR), which represents a contemporary means for preventing nitrogen oxide emissions. The oxygen content in the cylinder is reduced by exhaust gas recirculation and a reduction of the temperature in the combustion chamber results as a consequence thereof. The increase of the number of particles with increasing exhaust gas recirculation is problematic. The main reason for the higher particle emissions is the limitation of the oxygen, which is also required for the soot oxidation. The oxygen content, which is reduced by the exhaust gas recirculation, thus always has a decreasing effect on the nitrogen oxide emission and an increasing effect on the particle emission. A conflict of objectives arises therefrom between soot emissions and nitrogen oxide emissions, in particular in diesel engines.
Due to the previous statutory provisions for the exhaust gas test cycle, only low requirements were placed on the reduction of the pollutant emissions during dynamic operation for passenger automobiles. In the commercial vehicle sector, the dynamic operation was completely hidden by a stationary test.
The increasing requirements for modern diesel engines are characterized above all by the continuous tightening of the limiting values for pollutant emissions and by the introduction of new test cycles. These cycles will in future also consider the real driving operation within the scope of the certification for ascertaining realistic fuel consumption and emission values, which will be accompanied by a significant gain in dynamic response in the operational profile.
The dynamic driving operation and, linked thereto, the transient transitional behavior, will substantially enter the focus of further development and optimization efforts against this background.
The consideration of dynamic procedures requires in particular the consideration of load jumps or rapid load increases, as frequently occur in real driving operation and in future test cycles. Load jumps or rapid load increases result in a delayed buildup of the charge pressure due to the inertia of the air system in a diesel engine. The causes of this inertia are, inter alia, the moment of inertia of the turbocharger and the dead volume between the compressor and the intake valves of the engine. The injection system, which implements the load requirement of the driver, has a significantly shorter response time than the air system of the engine. A short-term and abrupt increase of the load, for example, an increase of the driver command torque, for example, during an acceleration procedure, therefore results in a system-related delayed torque buildup, which is reflected in sluggish response behavior of the diesel engine. This inertia is a consequence of the described behavior of the air system caused by dead times in the gas flow lines and due to the mass inertia of the compressor and results in a delayed charge pressure buildup in a reduced cylinder charge. This behavior displays emission-relevant effects in the air mass flow regulation, since, as a function of the transient driving state, a reduction of the EGR rate results in a massive increase of the NOx emissions. However, operation with a low air-fuel ratio along the smoke limit does not result in an increase of the soot emissions, but rather also in a limitation of the driver command torque at the expense of the drivability. In summary, it is thus to be stated that rapid load changes are expressed in a delayed torque buildup at the cost of drivability and in temporarily strongly increased emission peaks. This reflects the field of tension between driving performance and exhaust gas emission.